Moreover, the invention relates to an apparatus required for the implementation of this process. During the laser welding of plates and strips that, for example, are used in car body construction, the quality of the welding seam is of great significance. During butt welding the width of the gap is instrumental in assuring the quality of the weld. If the gap is too wide, the laser beam "falls through" or the weld quality deteriorates to an extent that scrap is produced. Due to the high volume of energy applied at the laser beam focus there is a risk that as a result of thermal expansion in the plate this applied energy causes a gap ahead of the focal point that severely impedes the quality of the welding seam. For that reason, cooling of the welding seam area is of great importance since an effective cooling will prevent gap formation.
During laser welding a cooling action by ambient air is well known as is the injection of cooling air through nozzles or the infeed of gases such as helium or argon. Furthermore, cooling methods involving clamping jaws or copper ledges have also been employed. For example, cooling a weld area with gaseous media during laser welding is known as prior art from EP 0 450 349.
The processes covered by prior art are partly unsatisfactory as far as their cooling performance is concerned, to some extent they require substantial expenditure for equipment. When the cooling process involves gases considerable gas volumes have to be blown onto the welding seam to obtain results that can viewed as fairly, if at all acceptable. If cooling of the weld seam area is to be effected by means of clamping jaws, e.g. for copper ledges (EP 01 89 806), these have to be carefully controlled and arranged. Moreover, cooling by clamping jaws is not feasible in the event of continuous welding processes.
From DE 40 10 077 a process is known for the continuous welding of abutting strips (plates) by means of a laser beam without adding filler metal. This process provides for the strips to be cooled in the area of the welding seam immediately after the welding focus so that the cooling intensity is controlled as a function of the width of the gap formed directly before the welding focus by the abutting edges of the strip in strip running direction in such a manner that this gap width remains within a tolerance range. From this process it is also known that cooling can be effected on both sides of the strip. For cooling, a gaseous medium is proposed, preferably air. To get as close as possible to the welding focus the narrow tip of the injection nozzles is envisaged to project into the gap between the clamping rollers. The blowing direction of the injected gaseous medium is away from the welding focus. Controlling the cooling intensity of the injected gaseous medium is achieved by way of its flow rate. In connection with this process an apparatus for the continuous welding of abutting strips (plates) by means of a stationary welding beam acting on the abutting strip edges is proposed that includes clamping rollers arranged in pairs vertically to the running strip direction at both sides of the strips to be welded, said clamping rollers forming a gap in the area of the joint of the strip through which the stationary laser beam hits the abutting edges to be welded. For this purpose a control apparatus is employed that has a measuring unit located in the direction of strip travel immediately ahead of the welding focus monitoring, as actual value, the width of the gap formed by the abutting strip edges and a cooling unit arranged in the direction of strip travel behind the welding focus and acting on the strips in the area of the weld seam, the cooling intensity of this unit being adjusted as a function of the actual value representing the gap width being constant within prescribed tolerance limits and being furnished by the measuring unit. The measuring unit consists of a radiating source arranged on one strip side and a radiation receptor arranged on the other. This radiating source can be a laser whereas the radiation receptor is a diode line camera.
Furthermore, the cooling unit has blowing nozzles for the gaseous medium. The gas cooling process requires a considerable consumption of gas and its cooling capacity is only limited.
DE 30 37 981 relates to a process for fusion and cutting welding, welding, sublimation cutting, drilling, marking as well as fine machining of materials by means of a laser beam with such beam being generated, directed, focused through a lens and projected onto the surface of the workpiece to be processed. Moreover, an inert or reactive gas is admitted to the focused spot thus generated coaxially to the laser beam. The focused spot can be displaced without an additional lateral correction of the workpiece coordinates and/or a nozzle opening of the laser beam in the direction of the laser beam axis relative to a fixed reference point on the axis, with the workpiece surface in the focused spot and in its immediate vicinity being cooled by a gaseous or liquid coolant stream concentrically applied to this surface. In this case the arrangement shall provide for a coolant of the workpiece in the form of at least one liquid jet located on an envelope of a cone coaxial to the laser beam the tip of which being located at least approximately in the focused spot.
From DE 41 33 350 a process is known as prior art that provides for the cutting of a prescribed pattern through the wall of a hollow metallic workpiece by means of a laser, with a liquid coolant being pumped through the workpiece supporting fixture and metallic workpiece so that the coolant passes through the workpiece during pattern cutting by means of a laser cutting apparatus and during this process contacts at least some part of the inner workpiece surface with a view to purging out slag particles that may form during cutting which minimizes or rules out an adherence of such slag particles to the inside of the workpiece. A water/oil mixture in the range of 50 parts of water and 50 parts of oil up to approximately 90 percent of water and 10 percent of oil, each based on volume, is to be used as coolant. When applying this liquid cooling method danger exists that liquid enters the focus or runs into a molten pool.